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The hydropeaking regime below hydropower facilities represents a serious threat to riverine fauna and may cause declines in populations living under its influence. However, the knowledge on direct fish responses to the threat of hydropeaking is limited. Here, we aimed to test whether the hydropeaking generated 12 km upstream may have a negative effect on the position of actively spawning rheophilic fish, asp, Leuciscus aspius. Two passive telemetry antenna arrays were used to record fish position on the spawning ground. We monitored the position of spawning fish (545, 764 and 852 individuals) in three one-month long spawning seasons in 2017–2019 and related the changes in detection probability on the two antenna arrays to flow conditions, temperature, time of a day and individual fish ID. The fish detection on the spawning ground was negatively affected by the flow change (both increase and decrease) in time. Moreover, the probability of fish detection was also influenced by water temperature, the time of the day and, as seen from the magnitude of individual random effect variability, the detection probability was rather individual-specific. Hydropeaking resulted in the change of spawning behaviour and likely caused interruption of spawning or shifting spawning outside the optimal area for egg development. We therefore advise to reduce the hydropeaking regime during the rheophilic fish spawning season under fisheries or conservation interests.

A novel sampling scheme, using a combination of electrofishing, visual exploration by scuba divers, two types of fyke nets and longlines, was tested in four reservoirs (including their inlets and outlets) to monitor a population of burbot Lota lota. This was supplemented by fry trawling and vertical hydro-acoustics, to detect L. lota larvae in two deep reservoirs that have had a long-term stocking programme. The majority of the L. lota detected were juveniles, captured by electrofishing in the littoral zones of the reservoirs and in running waters. Older individuals were rarely captured with longlines or fyke nets in deeper zones or structured habitats within the reservoirs. A combination of multiple sampling methods provided an assessment of the whole population. Population establishment could not be demonstrated as the age structure of the sampled fish corresponded with that of the stocked fish. Low post-stocking survival, migratory behaviour, interactions with other species and warmer water temperatures are considered the potential drivers for unsuccessful establishment of L. lota populations in these reservoirs.

The jute mattings of two different densities were tested to control growth of Elodea canadensis Michx. and Myriophyllum spicatum L. in the Velký Bolevecký pond (West Bohemia, Czechia) during fourteen months. Both jute densities prove to be effective, permeable and stayed intact for one year. Results showed significant interaction among species, jute density and date in both abundance and length of Elodea and Myriophyllum fragments. When tested separately, we found the significant difference between abundance of Elodea and Myriophyllum fragments on the jute mattings, among the sites, and in time, but not between the two jute densities. Elodea dominated the sites due to spread of fragments from mowing that continued at the pond. However, the fragment length of given species varied between the two jute densities. They might easier root, but it could be also explained by the variability of sampled quadrats. Fragment lengths of Elodea and Myriophyllum were also changing in time but the length did not significantly differ between the two species or among the sites. Although Elodea and Myriophyllum dominated the jute mattings by the end, their progress was slower than in control sites.

Fishponds exhibit high natural retention potential for phosphorus, which enters the ponds from non-point, diffuse and point sources, as well as from aquaculture management. Results of phosphorus mass balance monitoring of nine large fishponds (60–449 ha) over 2010–2014 revealed total phosphorus retention ranging between −66 % (sink) and +52 % (release) of the P loads from inflows, i.e. specific P retention varied between −7.83 g m−2 (sink) and 1.00 g m−2 (release) of surface area, per one fish production cycle. Retention of P was eliminated by application of fertilizers (with simultaneous fish feed application) during fish production process and/or by massive P release from fishpond sediments after previous heavy loads. P retention could be increased by preference of surface water outflow instead of bottom discharge and/or also by minimizing of emissions of suspended sediment particles during a fish harvesting. The real role of fishponds in transport processes of P throughout a watershed could be eruditely assessed only by following newly proposed method called “new approach” when P input (via inflows) and P output (via outflows) are compared with natural fishpond ability to retain P, which is determined by a simple model proposed by Hejzlar et al. (2006). For evaluation of effect of actual fishery management, the “culture-balance” method is suitable too. If the P inputs (stocked fish, fertilizing, feeding) and outputs (harvested fish) derived from fishery practice were in balance, the P retention did not decline. However, in condition of inputs > outputs, the P retention appreciably ceased or was totally eliminated. It was concluded that fishponds could serve as an important sink of P transported throughout a watershed even under semi-intensive fish (common carp) production condition.

Promotion of successful examples of biomanipulation and long-term monitoring is important for encouragement of similar activities, which are still relatively scarce. A project to improve water quality in the Velky Bolevecky pond (Plzeň, Czech Republic) began in 2006. The main activities in this project were: large scale fish reduction; stocking of predatory fish; usage of phosphorus binding coagulants; and reintroduction of macrophytes. During five years the impact of a eutrophic system with massive cyanobacterial blooms with low water transparency transformed into a clear water system with high water transparency and well developed macrophyte cover. All aspects relating to the ecological status of the water body such as total phosphorus concentration, chlorophyll-a concentration and cyanobacterial and other phytoplankton density improved significantly during or immediately after the project realization. For example, water transparency increased three fold. Zooplankton density also increased, however the difference was not significant. Oligotrophication of the system became evident also in the species composition of zooplankton and phytoplankton. Species adapted to nutrient reduction and macrophyte cover developed. Seven years after the project was over, the fish stock remained on the low densities naturally. A good ecological stage was achieved with low nutrient levels, high water transparency, very low densities of cyanobacteria and other phytoplankton together with a well-developed macrophyte cover. All of which has been maintained without any expensive impact. The highest expenditure is, paradoxically, due to macrophyte harvesting because their massive explosion in the pond is not in accordance with recreational and sporting utilization. It seems, that reduction of nutrient loading and switching of a formerly eutrophic system to an oligotrophic one is possible and sustainable over a relatively long time period in a shallow temperate pond. Subsequent good management of the system will, of course, be necessary.

Series of 16 fishponds was monitored as a part of a drainage basin of Hracholusky reservoir, which is subject of a recovery project. All the fishponds were eutrophic – hypertrophic due to wastewaters from villages and little cities (including overflows from sewers) and/or due to too intensive production of “hunters ducks” – the phenomenon that is rarely mentioned.

The estimation of age- and sex-specific mortalities is essential for constructing fish life tables and subsequent population modelling. However, the ecological data behind these parameter estimates acquired in nature may be systematically biased by the methodology of the study. To demonstrate mark retention bias by the widely used passive integrated transponder tags (PIT tags), we analysed long-term data showing the probability of tag retention within the body cavity in relation to fish sex. We used the cyprinid fish asp (Leuciscus aspius) as a model species with a cystovarian type of ovulation (eggs do not enter the body cavity). Altogether, 2312 fish were tagged with PIT tags in 2014–2018 and fin clipped and monitored for up to five years. In this period, 583 asp were recaptured at least one year after tagging, and fish that lost their tags were identified by regenerated fin. Based on recaptured fish, we show that the female tag retention rate is significantly lower than the male retention rate (85.3% in females vs. 97.8% in males). Furthermore, we used portable antenna to detect expelled PIT tags and localized a higher number of expelled female PIT tags on the spawning ground in comparison with those from males (123 female vs. 6 male PIT tags). This study demonstrates that systematic bias may potentially occur since PIT tags may penetrate gonads and leave the body during reproduction. We encourage designing PIT tag monitoring studies with respect to potential unequal sex-specific tag retention capabilities to avoid drawing inaccurate conclusions from biased data.

Piscivorous fish may drive trophic cascades and maintain good water quality in lakes and reservoirs. Biomanipulation by stocking piscivores has been therefore suggested as a useful tool to regulate phytoplankton biomass in anthropogenically eutrophicated systems. We evaluated the top-down effects of piscivores on lower trophic levels in 13 deep, stratified Czech reservoirs that were built by damming rivers. In addition, the importance of piscivores was assessed in three deep, bank-side Dutch reservoirs that are artificially destratified. All the Czech reservoirs are regularly stocked with piscivores while the Dutch reservoirs have never been stocked. The piscivores had no significant top-down effect on the biomass of planktivorous and benthivorous fish and on phytoplankton biomass (measured as chlorophyll-a concentration) in the Czech reservoirs, although the proportion of piscivores in the fish community was high in half of them. The planktivore and benthivore biomass and the phytoplankton biomass correlated strongly with phosphorus concentration, which is in line with bottom-up control by nutrient supply. On the other hand, piscivores in the Dutch reservoirs dominated the fish community and apparently contributed to good water quality. Nevertheless, the primary cause of relatively low phytoplankton biomass in the Dutch reservoirs was the artificial destratification inducing light limitation on phytoplankton. We conclude that biomanipulation cannot substantially reduce phytoplankton biomass in the eutrophicated systems with high external nutrient input. The input should be reduced to control phytoplankton and if it is not possible, artificial destratification can be applied.

High eutrophication is the main factor that has recently disabled the Jordan water reservoir for water supply and recreational purposes. Large-scale dredging of sediments was done between years 2011 and 2014. Thereafter a complex monitoring of water quality has been initiated since 2015.

Stratification and phosphorus fluxes (input, output, sedimentation, and release from sediments) were studied in Ceske udoli Reservoir (49°43'N, 13°21'E; V - 2.65 × 10 6 m 3 ; A - 1.04 × 10 6 m 2 ; z max - - 5.5 m; surface altitude - 314 m a.s.l.) during two summer stratification periods which differed in outlet operation and in hydraulic residence time (1997: surface outlets and 14 days; 1998: bottom outlets and 23 days). Use of bottom outlets resulted in weaker thermal stratification, a less pronounced oxygen deficit in the hypolimnion, and significantly lower P retention (17%) in comparison with surface discharge (42%). Factors apparently contributing to lower retention of P during the use of bottom outlets were: (i) faster flushing of the hypolimnion which intensified longitudinal transport of particles, (ii) more intense P release from sediments due to a lower concentration of nitrate in the water column and to more frequent contact of sediments in the shallows with water of pH > 9. During both periods, phytoplankton production was limited only by light, not by nutrients. Phytoplankton biomass was comparable in both years despite some differences in vertical distribution.